1. Field of the Invention
The present invention relates to explosion proof vaults which are typically employed to house electrical equipment in areas which may be exposed to explosive fumes.
2. Description of the Prior Art
In commercial and military aviation ground support facilities aircraft are serviced on the ground at aircraft terminals, remote parking locations, or at maintenance bases. The ground support functions which must be performed at such locations include refueling, the provision of electrical power to the aircraft while it is on the ground, the provision of cooling air to enhance cabin comfort, and the provision of pressurized air to start the aircraft engines. Such ground support servicing functions are performed at modern air terminals using umbilical cords, lines and conduits which are housed within prefabricated pits when aircarft are not present on the ground in the vicinity. Such prefabricated pits are installed at aircraft fueling and loading areas. The pits are enclosures which are installed below the surface of loading and refueling aprons, at aircraft terminals, remote parking locations, and maintenance bases.
The prefabricated pits house valves, junction boxes, cooling air terminations and other terminal equipment used to service aircraft that have been docked. Umbilical pipes and lines, otherwise housed within the pits, are withdrawn from the pits through hatches and are coupled to the docked aircraft to supply it with fuel, air and electrical power. The use of such subsurface pits eliminates the need for mobile trucks, carts and other vehicles which are otherwise present in the loading area and which interfere with the arrival and departure of aircraft at a loading gate.
As a safety precaution all ground support equipment items in such prefabricated pits which employ electrical contacts must be housed within explosion proof enclosures due to the presence of vaporized fuel in the vicinity. The presence of vaporized fuel in air can lead to explosive gaseous mixtures. Such mixtures could be ignited by arcing electrical contacts unless such contacts are housed in an explosion proof vault.
In an explosion proof enclosure, the casing forming the enclosure is not completely airtight. If it were, an arcing electrical contact in an explosive atmosphere would ignite the explosive vapors and cause the casing to shatter. In an explosion proof enclosure the interface between the strong, heavy casing sections is sufficiently long, and sometimes tortuous, so that any gaseous mixture ignited within the casing will travel a sufficient distance across the interface and will cool before escaping the enclosure. The path of escape of the expanding gas at the interface is long enough so that the ignited gas cools sufficiently to extinguish any flame. Thus, despite ignition and explosion within the enclosure, no flame will escape the enclosure. As a result, no gas outside of the enclosure will be ignited as a result of an explosion within the enclosure.
One problem which exists within conventional explosion proof casings is that the enclosure "breathes" with changing ambient temperature. The enclosure is cool in the early morning hours, perhaps 60.degree. F. With increasing heat of the day the temperature rises. In some locations the interior of the enclosure reaches a temperature of as much as 160.degree. F. The air in the enclosure expands by a volume of about 17% for such a temperature change. The expanding air escapes through the casing interface.
As the enclosure cools in the late afternoon and evening, air containing moisture is drawn into the enclosure through the casing interface. The moisture in the air condenses in the enclosure. With the passage of time, the moisture builds up in the enclosure because the ambient air drawn in is usually warmer than the air inside of the enclosure. That is, air laden with water vapor is drawn into the enclosure, while relatively dry air is expelled therefrom. The moisture remaining in the enclosure attacks the control mechanisms housed therewithin and corrodes relay and terminal contacts and otherwise disrupts the operation of the encased equipment.